The invention relates to a formwork panel for concrete walls comprising a forming skin supported by a steel frame in which a bore extending through the steel frame and arranged in a bushing is provided for receiving a tie rod extending through the wall. In known formwork panels the steel frames consist of rectangular hollow sections arranged along the edges of the forming skin which generally takes the form of a plywood panel. The forming skin is further supported by medium struts and cross-struts. Along the edges of the formwork panel there are provided openings for receiving the tie rods. As a rule, these openings take the form of bores extending through the forming skin, the wall of the hollow section supporting the forming skin and also the opposite wall of the hollow section. The openings into which no tie rod is inserted must be covered with a plastic cap or the like before the concrete is poured to prevent the concrete from entering these bores and hardening therein. Experience shows, however, that the openings not used for the insertion of tie rods remain uncovered for numerous reasons, either because the caps are not readily at hand, or else for other reasons.
Then, when the formwork panels are re-used, it will first be necessary in many cases, before the tie rods can be inserted, to remove the hardened concrete from the openings with the aid of a concrete drill. This is not only a troublesome and time-consuming work, but also a handicap which seriously disturbs the assembly operation. It has already been known to make the openings through the steel frame and the forming skin a little larger and to weld a bushing into the opening which will prevent the concrete from penetrating into the hollow interior of the rectangular section of the frame. However, the removal of the concrete plug hardened in this steel bushing is by no means facilitated by this arrangement, but rather rendered more difficult because of the adhesion of the hardened concrete plug to the inner wall of the steel bushing.
Now, it is the object of the present invention to facilitate the removal of any concrete that may have penetrated into bores of this type and hardened therein.
According to the present invention, this problem is solved by a bore of conical shape provided in a bushing made of a plastic material extending through and fastened in the frame.
The conical shape of the bore and the fact that the bushing is no longer made of steel, as in the known arrangements, but of a plastic material, reduces the adhesion of the hardened concrete in the bore so that the concrete plug can be easily pushed out of the bore from the end of the bore having the smaller diameter. A hammer and if necessary a rod may be used, it is no longer necessary to use a drilling machine for the cleaning of such tie rod bores. The material used for the manufacture of the plastic bushing should have an impact resistance high enough to prevent damages to the bushing as a result of the removal of the concrete plug. Moreover, the slightly conical bore of the plastic bushing should have a smooth interior surface, but this is a feature anyway exhibited by most of the existing plastic materials. Although the term "bore" is used in this specification in connection with the plastic bushing, it does not mean that the slightly conical central opening in the bushing is produced by drilling. Rather, these bushings are produced in the usual manner by die-casting or the like.
The material of which the plastic bushings are made may preferably consist of a polyamide, but of course any suitably adjusted PVC may also be used.
In one embodiment of the invention, the plastic bushing comprises a mounting flange engaging a projection of the forming skin from behind. The projection is preferably formed by a recess in the side of the forming skin facing the frame, and the flange is fixed between the forming skin and the frame over at least a portion of its periphery. This offers the advantage that the plastic bushings can be fastened in the forming skin by very simple means, i.e. by inserting it into a bore in the frame of a corresponding size before mounting the forming skin on the steel frame, so that after the assembly of the forming skin, which generally consists of a plywood panel, to the frame the flange of the plastic bushing is clamped between the forming skin and the frame and, thus, retained in position. To this end, the flange need not necessarily engage a recess in the side of the forming skin facing the frame. Rather, the plastic bushing can be fixed also by causing the flange to engage either a slot in the forming skin extending in parallel to its plane, or else a recess in the face of the forming skin facing the concrete. In this latter case, the bushing must be fastened to the forming skin by nails or the like applied through the flange. The flange serving to fix the plastic bushing in position need not necessarily engage the forming panel about the whole periphery of the plastic bushing. Rather, the bushing will be sufficiently fastened also when the flange is engaged by a projection of the forming skin only about a portion of its periphery.
In another embodiment of the invention, the conical bore of the plastic bushing has its larger diameter at the end of the bore opposite the forming skin. The main advantage of this arrangement lies in the fact that any concrete entering the open end of the bore situated in the forming plane will encounter an increasing cross-section as it penetrates into the bore and apply only little pressure upon the inner faces of the plastic bushing so that, as a result, there is only little risk that the angle of taper of the bore may be disturbed at some point by an expansion of the plastic bushing. As the taper of the bore remains unchanged even in cases of extended use of the formwork panel, the hardened concrete plug will always be in loose contact only with the bore so that it can always, even in the case of extended use of the panel, be easily removed, if necessary with one single blow of the hammer. In addition, this embodiment offers the following advantage: Tie rods are as a rule inserted only after one side of the formwork of the concrete wall has been erected and when the opposite formwork is being assembled. In this case, the worker works on the inner side of the formwork first erected. If in such a case he wishes to remove a concrete plug from a bore in a formwork panel first erected, it is a definite advantage if this can be done by him without changing his position, i.e. from the inner face of the formwork first errected. Another advantage of this embodiment lies in the fact that the spacer sleeve applied on the tie rod covers the smaller hole.
In order to give the plastic bushing increased strength and to safely prevent, for instance any expansion of its bore in the course of time, the plastic bushing may be embedded in a supporting bushing made from metal, preferably iron or steel. In one embodiment of the invention, this supporting bushing surrounds only the portion of the plastic bushing situated in the steel frame, whereas in other embodiments the supporting bushing may surround also the portion of the plastic bushing situated in the forming skin, in addition to that situated in the frame.
In one embodiment of the invention, the supporting bushing surrounds only the portion of the plastic bushing situated within the hollow section. In this case, the supporting bushing is retained by beading over the edges of the bores in the walls of the hollow section. As compared to this arrangement, the metal bushings inserted in known formwork panels into the bores of the steel frames are retained therein be welding seams or welding spots. This arrangement offers the disadvantages that the larger bore weakens the section in the area of these welds, that deformations are caused by the welding operation and that welding is more expensive.
The above-mentioned embodiment of the invention may be further improved by an arrangement in which the diameter of the supporting sleeve is enlarged at the sleeve ends and in which the beaded edges of the bores provided in the walls of the frame fit into the space between the enlarged ends of the supporting bushing and the outer face of the plastic bushing.
In this arrangement, the enlargement preferably corresponds to the thickness of the beaded edges so that the plastic bushing is also guided at this end. Or else the interior diameter of the supporting bushing can be left unchanged whereas the outer diameter of the plastic bushing is sufficiently reduced in the area of the ends of the supporting bushing so that a clearance is obtained between the inner wall of the supporting bushing and the outer wall of the plastic bushing for receiving the beaded-over edges of the bores.
In known formwork panels, the bore in the steel frame is continued, with the same diameter, in the forming skin mounted on the frame. This proved disadvantageous because only a thin web of the forming skin, which consists of a plywood panel, remains between the outer edge of the forming skin and the section of the bore nearest to the edge, which web tends to break after repeated use of the formwork panel. In one embodiment of the invention, the opening intended for receiving the tie rod, which must be in alignment with the bores in the frame, is no longer drilled, but produced by means of a milling cutter which is applied to the edge of the forming skin and advanced inwardly until the axis of the milling cutter coincides with the axis of the bore in the frame. Accordingly, the opening receiving the tie rod comprises two straight lateral faces extending inwardly from the edge and ending in a semicylindrical surface which is in alignment with the bore in the frame. According to the invention, the end of the plastic bushing which is flush with the forming plane is provided with a head matching the shape of such an opening produced with the aid of a milling cutter. The advantage of this arrangement lies in the fact that any breaking of the forming skin at its edge adjacent to the bore is prevented and that the forming skin exhibits an even surface in the forming plane also at this point.
The invention may be realized also by an arrangement in which a formwork panel has inserted into openings in the steel frame metal bushings for receiving the tie rods which have the inner wall of their bore lined with a plastic layer so that essentially the same advantages are obtained as in the case of the embodiment of the invention in which the plastic bushing takes the form of a separate part. The bore in the metal bushing increases slightly conically towards the end of the bushing opposite the forming skin, and the plastic layer is of essentially constant thickness over the length of the bore. The bore of the metal bushing may also be of cylindrical shape and have pressed into it a plastic bushing having a conical bore, the outer face of the plastic bushing being provided with a torus engaging an annular groove provided at the inner face of the metal bushing.